add support for Configuration API

[dmitsh]

The Configuration API feature has been implemented in Dapr runtime: dapr/dapr#3606
It needs to be accessible via the SDK.

[withinboredom]

This is only supported via GRPC, so it can't be supported by the PHP SDK until available via HTTP and GRPC.

[msfussell]

An http API is now added in the v1.8 release.

[withinboredom]

Woohoo!